Question

I have two tables with a linking table describing their many-to-many relationship.

TABLE buildings
    building_id
    building_name

TABLE facility_types
    facility_type_id
    facility_type

TABLE buildings_2_facility_types
    building_id
    facility_type_id

Now I need a report including the facility types as columns per each building.

REPORT:

  NAME        golf    pool    garage
Building A     Y        N        Y
Building B     N        Y        Y
Building C     N        N        N

How can I do this in SQL?

N.B. This will be part of a much larger query involving lots of other tables. I can do all that part, but converting rows to columns has got me vexed!

Answer 1

Converting rows to columns is a real pain and you have to understand that SQL is really not meant for this kind of operation.

It’s still possible, providing that the set of values to be transposed is small, known and limited. If not, you’ll have to rely on you application layer.

An exemple on how to do it should help you understand the 3 conditions:

Assuming a table buildings_2_facility_types:

building_id facility_type_id
    1               1
    1               2
    2               3
    3               1
    3               3

You can get sthg out of it this way:

SELECT
    building_id,
    IF(SUM(facility_type_id = 1) > 0, "Y", "N") as facility_type_1,
    IF(SUM(facility_type_id = 2) > 0, "Y", "N") as facility_type_2,
    IF(SUM(facility_type_id = 3) > 0, "Y", "N") as facility_type_3
FROM buildings_2_facility_types
GROUP BY building_id;

See ? small, known and limited :/

Of course, you can also do it with a proc stock but it’s usually not the prefered way.